Refrigeration defrost system

ABSTRACT

A defrost system for refrigeration apparatus in which operation of the defrost system is initiated by frost build-up on the evaporator of the refrigeration apparatus. The defrost system includes a conduit having one end thereof constituting an outlet in close proximity to the evaporator for directing a stream of air against a portion of the evaporator. Upon build-up of frost on the evaporator, the outflow of air from the conduit is partially restricted by the frost, this restriction causing a flow of chilled air to be directed onto a defrost initiation thermostat thereby to cold-bias the thermostat and to initiate the defrost cycle.

United States Patent [191 Blanton Mar. 25, 1975 REFRIGERATION DEFROST SYSTEM Primary E.\'aminerMeyer Perlin Attorney, Agent, or Firm-John A. Haug; James P. [75] Inventor. Bobby D. Blanton, Garland, Tex. McAndrewS; Edward J. Connors, Jr. [73] Assignee: Texas Instruments Incorporated,

D' ll T ex 57 ABSTRACT Flled' Sept' 1973 A defrost system for refrigeration apparatus in which [21] Appl. No.: 394,949 operation of the defrost system is initiated by frost build-up on the evaporator of the refrigeration appara- [52] U S Cl 62/140 67/156 tus. The defrost system includes a conduit having one [5 H Fzsd 51/02 end thereof constituting an outlet in close proximity to [58] Fie'ld "62/140 156 the evaporator for directing a stream of air against a portion of the evaporator. Upon build-up of frost on the evaporator, the outflow of air from the conduit is [56] References Cited partially restricted by the frost, this restriction causing UNITED STATES PATENTS a flow of chilled air to be directed onto a defrost initi- 2,947.l53 8/l960 Atchison 62/140 ation ther ostat {hereby t0 cold-bias the thermostat /1 and to initiate the defrost cycle.

11 Claims, 5 Drawing Figures PATENTED HAR 2 51975 FIC5.I

REFRIGERATION DEFROST SYSTEM BACKGROUND OF THE INVENTION This invention relates to a defrost system for various types of refrigeration apparatus, the defrost system being operable in response to the build-up of frost on the cooling unit (e.g., the evaporator) of the refrigeration apparatus.

The accumulation or build-up of frost on the evaporator of a refrigerator or other refrigeration unit has long been a problem. Various automatic defrosting systems have been used and are well known in the art. Typically, an automatic defrost system is controlled by a timer which initiates operation of the defrost system at certain times of the day or after the compressor has run a predetermined length of time. The rate at which frost forms on the evaporator is a function of the amount of water vapor in the air passing over the evaporator, the greater the water content the faster the frost accumulates. In a refrigerator, the amount of water vapor within the air to be cooled depends a great deal on the ambient conditions (i.e., room temperature and relative humidity) outside the refrigerator because ambient air is introduced into the refrigerator each time the door is opened and closed, the frequency with which the door is opened and closed, and water vapor sources (e.g., wet produce and open containers of liquids) within the refrigerator. With time-controlled defrost systems and with a slow build-up of frost, operation of the defrost system is sometimes initiated before any significant amount of frost has built up on the evaporator, thus resulting in a wastage of power to defrost the refrigerator when it is not required and exposing the items in the refrigerator to unnecessary defrost cycles. On the other hand, under heavy frost conditions, excessive frost may build up on the evaporator between the timed defrost cycles, thus reducing the efficiency of the refrigerator and increasing the power consumed thereby.

SUMMARY OF THE INVENTION 7 Among the several objects of this invention may be noted the provision of an automatic defrost system for refrigeration apparatus (e.g., a refrigerator, a freezer, a refrigerated vending machine, or an air conditioner) in which build-up of frost on the evaporator of the refrigeration apparatus initiates a defrosting cycle rather than having defrosting initiated on the basis of time; the provision of a demand defrost system which conserves power and increases the operating efficiency of the refrigeration apparatus by eliminating unnecessary defrost cycles and by keeping the refrigeration apparatus free of excessive frost; the provision of such a defrost system which does not expose refrigerated or frozen items to unnecessary defrost cycles; the provision of such a defrost system which is relatively simple and of economical construction and which will reliably operate regardless of ambient climatic conditions. Other objects and features will be in part apparent and in part pointed out hereinafter.

Briefly, a defrost system of this invention is intended for use on refrigeration apparatus having cooling means for absorbing heat from a refrigerated zone, the cooling means being subject to frost build-up. The refrigeration apparatus has a flow path for the intake of air from the refrigerated zone, for passage of the air over the cooling means thereby to chill the air and for the discharge of the chilled air into the refrigerated zone, and a blower for forcing the air through the flow path. The defrost system of this invention comprises a heater for removing frost from the cooling means, and means for initiating operation of the heater in response to the build-up of frost on the cooling means. This initiating means includes a conduit providing communication between the cooling means and the flow path, on the discharge side of the blower, one end ofthe conduit being in close proximity to a portion of the cooling means, the blower forcing chilled air through the conduit for outflow of air from the one end of the conduit and for impingement of the outflow on the abovementioned portion of the cooling means. Means is also provided for sensing air temperature adapted to ener-.

gize the heater upon sensing a predetermined depressed temperature of the air. Means responsive to restriction of the one end of the conduit by build-up of frost on the cooling means directs a flow of chilled air onto the sensing means for lowering the temperature of the air sensed thereby to the predetermined depressed temperature, this last-said means including a passage for directing the flow of chilled air onto the sensing means.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view in section of refrigeration apparatus employing a defrost system of this invention and illustrating the normal flow of air therethrough when the refrigeration apparatus is in operation;

FIG. 2 is a view similar to FIG. 1 illustrating an alternate flow of air therethrough upon the build-up of frost on the cooling unit of the refrigeration apparatus thereby to initiate operation of the defrost system;

FIG. 3 is an enlarged view of a portion of the refrigeration apparatus illustrating a conduit having its free end in close proximity to a refrigerant line constituting a portion of the cooling unit of the refrigeration apparatus and having a tube disposed therewithin with arrows depicting air flowing in the conduit and tube impinging DESCRIPTION OF A PREFERRED EMBODIMENT Referring now to the drawings, a defrost system of this invention, as indicated generally at 1, is shown installed in a conventional two-door refrigerator-freezer 3. The refrigerator-freezer includes a cabinet 5 having a top 7, side walls 9, a back wall 11, a bottom wall (not shown) and apartition l3 dividing the interior of the cabinet and defining a freezer compartment 15 and a food compartment 16, these compartments constituting refrigerated zones. A freezer door 17 and refrigerator door 19 close the front of the cabinet. The refrigerator includes a conventional refrigeration system including a compressor driven by an electric compressor motor 21 (see FIG. 5), a condenser (not shown) and a cooling unit or evaporator generally indicated at 23. The evaporator includes a plurality of refrigerant lines 25 constituting a coil, this coil being subject to frost build-up (see FIGS. 2 and 4). A flow path generally indicated at 27 provides for the intake of air from both the freezer and food compartments, for the passage of this air over the evaporator for absorbing heat from the air and thus chilling the air, and for the discharge of the chilled air into the refrigerated compartments or zones. A blower or fan 29 is provided for forcing air through the flow path. While the defrost system of this invention is depicted installed in a two-compartment refrigerator-freezer, it will be understood that it may be installed in other refrigeration apparatus, such as a single-compartment refrigerator, a freezer, a refrigerated vending machine, or an air conditioner.

More particularly, flow path 27 is, in part, defined by partition 13 and by a horizontal panel 31 in freezer compartment spaced above the partition and thus forming a main passage 33 between the horizontal panel and the partition. An opening 35 is provided in partition 13 for the intake of air into the passage from food compartment 16 and an opening 37 is provided in panel 31 for the intake of air from the freezer compartment. Evaporator 23 is located within passage 33 for chilling air from the food and freezer compartments as it passes thereover. An inner vertical panel 39 is spaced from backwall 11, thereby to provide a return or outlet passage 41 for the discharge of chilled air into the food compartment via an outlet 43. A vertical wall 45 extends up from panel 31 and a fan shroud 47 is disposed between vertical wall 45 and panel 39, thereby to define a fan intake chamber 49 and a discharge chamber 51, with the upper end of the fan intake chamber being closed by a cap 53. An opening 55in panel 31 provides communication between main passage 33 and the fan inlet chamber. A baffle 57 directs and divides the chilled air discharged from the fan into outlet passage 41 for discharge into the food compartment and into freezer compartment 15 via openings 58 and 59.

The defrost system 1 of this invention comprises a heater H for melting and removal of frost F from evaporator 23 and means, as indicated generally at 61, for initiating operation of the heater in response to a predetermined build-up of frost F (see FIG. 4) on the evaporator. Means 61 is shown to comprise a conduit 63 extending between discharge chamber 51 and main passage 33 with one end of the conduit constituting an outlet 65 in close proximity to one of the lines of evaporator 23 and the other end of the conduit constituting an inlet 67 connected to a port 69 of discharge chamber 51. It will be understood that while conduit 63 is illustrated as a tube, the term conduit may refer to any air passage (e.g., a duct, a closed channel, a passageway, etc.) which provides communication between the discharge side of fan 29 and the cooling means (e.g., the evaporator).

As viewed in FIG. 3, outlet end 65 of conduit 63 is separated from the adjacent evaporator refrigerant line 25 by a gap G. This gap may be varied, but its dimensions (e.g., A; inch) are such that as air outflows from the end of the conduit, the build-up of frost F on the refrigerant line 25 causes a reduction of the gap and thus restricts the flow from the conduit. An enclosure 71 in food compartment 16 houses a defrost initiation thermostat TSI. Enclosure 71 has an opening 73 therein for communication between the interior of the enclosure and the food compartment. Thermostat TSl constitutes air temperature sensing means and it is electrically connected, as shown in FIG. 5, to heater H to energize the heater upon sensing a predetermined depressed temperature (approximately +l5F.) within enclosure 71. A second conduit or venturi tube 75 extends from partition 13 adjacent thermostat TSl into flow path 27 and provides a passage for communication between enclosure 71 and flow path 27. More particularly, tube 75 is of smaller cross section than conduit 63 and it extends into the conduit intermediate the ends thereof. As shown in FIGS. 1 and 2, the free end oftube 75 extends longitudinally within conduit 63 and faces toward the outlet end 65 thereof.

Referring now to FIG. 5, compressor motor 21 is connected in a series circuit including the normally closed contacts of defrost initiation thermostat T51 and the contacts of a temperature control thermostat (cold control) TC across electrical power supply lines L1 and L2. Fan 29 is shunt-connected across the compressor motor 21 so that these two units are concurrently energized and de-energized. Cold control thermostat TC is positioned within the refrigerator in a selected location (not shown) to sense and regulate the temperature of the two compartments 15 and 16. The defrost initiation thermostat TSl, when sufficiently cooled, is actuated to its alternate position to disconnect the fan and compressor and supply power to heater H and a defrost termination thermostat TS2. Both thermostat switches T81 and T82 are of the wide differential type, the initiation thermostat being actuated from its FIG. 5 position to complete the heater circuit at about 15F., for example, and remaining in that mode until TSl is warmed to a substantially higher temperature, e.g., 4045F. The switch contacts of defrost termination thermostat T52 will remain closed as shown in FIG. 5 until its temperature rises to about 65F, these contacts remaining open until TS2 recools to a substantially lower temperature, e.g., 0l5F.

With refrigerator 3 operating in its normal refrigeration mode, the contacts of cold control thermostat TC and those of defrost termination thermostat TS2 are closed and the contacts of defrost initiation thermostat TSl are as shown in FIG. 5 thereby energizing compressor motor 21 and fan 29 and maintaining heater H1 deenergized. With fan 29 operating and with evaporator 23 substantially free of frost build-up, the fan draws in air from the refrigerated zones (e.g., from the freezer and food compartments 15 and 16, respectively) into main passage 33 by means of openings 35 and 37. This air passes over evaporator 23 which absorbs heat from the air and chills the air. Upon contact with the evaporator, excess water vapor in the air condenses on the evaporator and forms frost (see FIG. 2). The rate at which frost builds up on the evaporator depends upon the moisture content in the air to be chilled and the relative lengths of the on and off cycles of compressor motor 21 and fan 29.

More particularly, fan 29 draws the chilled air from the main passage into fan intake chamber 49 via opening 55. The fan discharges the chilled air into discharge chamber 51 and most of the air is discharged into freezer compartment 15 via outlets 58 and 59 and into food compartment 16 via passage 41 and outlet 43. A portion of the chilled air in the discharge chamber is forced through port 69 into the inlet end 67 of conduit 63. The flow of chilled air through the conduit from its inlet end to its outlet end 65 creates a partial-vacuum or suction at the free end of venturi tube 75 and thus causes air to flow from the food compartment into conduit 63 via opening 73, enclosure 71 and tube 75. The flow of chilled air from the inlet end of conduit 67 is mixed with the relatively warm air from the food compartment, and this mixed flow impinges upon one of the loops of evaporator 25, as shown. in FIG. 3. The relatively warm air from the food compartment contains more moisture than the dry, chilled air from the discharge chamber, and thus the mixed stream of warm and chilled air impinging on this portion of evaporator 25 causes an accretion of frost to build up thereon at a rate generally commensurate with the rate at which frost builds up on the balance of the surfaces of evaporator 25 due to the passage thereover of return air from freezer compartment and food compartment 16.

As shown in FIG. 4, upon the build-up ofa predetermined amount of frost F on the portion of evaporator 25 adjacent to the outlet end 65 of conduit 63, the gap therebetween becomes partially blocked, as indicated at G. This blocks the venturi-type action and increases the back pressure in the conduit as the outflow of air from the conduit is reduced. This reverses the direction of flow of air in tube 75 and causes chilled air to flow into enclosure 71. This inflow of chilled air into the enclosure causes a reduction of air temperature therein which cold-biases thermostat TSl. Upon TSl reaching a predetermined depressed temperature (e.g., approximately +15F.) it is actuated from its FIG. 5 position to de-energize compressor motor 21 and fan 29 and energize defrost heater H thereby to start the defrost cycle. As the cold control thermostat TC will be closed because of the somewhat increased box temperature, heater H will remain on until defrost termination thermostat TS2 senses a rise in temperature within main passage 33 to a predetermined defrost termination temperature (e.g., approximately 65F). Heater H. May, for example, be in its defrost heating mode for about 10 minutes. Due to the fact that the temperature of T81 is depressed, its temperature lingers below the compressor initiation temperature (e.g., approximately 42F) after the back flow of chilled air via tube 75 has stopped. It may, for example, take -30 minutes for the temperature of TSl to rise to its compressor initiation temperature because, without fan 29 operating, there is little or no flow of air into or out of enclosure 71 via opening 73. Due to the increased temperature in passage 33, melted frost will drain off while TSl warms to its compressor initiation temperature. When thermostat TSl warms to the compressor initiation temperature the defrost cycle is terminated and heater H is disconnected, compressor motor 21 and fan 29 are reenergized, and the refrigerator again assumes its normal mode of operation.

It will be particularly noted that the defrost cycle is initiated only in response to a build-up of a predetermined amount of frost on evaporator coils 25, and accordingly, the frequency of defrost cycles is minimized and the evaporator is kept free of excessive frost. Thus, the power consumed by a refrigerator incorporating the demand defrost system of this invention is substantially reduced by elimination of unnecessary defrost cycles and by keeping the evaporator free of excessive frost build-up, thereby to maintain a high heat transfer efficiency of the evaporator.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. A defrost system for refrigeration apparatus having cooling means for absorbing heat from a refrigerated zone, said cooling means being subject to frost buildup,

said refrigeration apparatus having aflow path for the intake of air from said refrigerated zone and for passage of the air over said cooling means to chill the air and for the discharge of the chilled air into the refrigerated zone, and a blower for forcing the air through said path; said system comprising a heater for removing frost from said cooling means, and means for initiating operation of the heater in response to buildup of frost on the cooling means, said initiating means comprising a conduit providing communication between said flow path on the discharge side of the blower and said cooling means, one end of said conduit being in close proxjmity to a portion of said cooling means, said blower forcing air through said conduit for outflow of air from said one end thereof and for impingement of said outflow on said portion of the cooling means, means for sensing air temperature adapted to energize said heater upon sensing a predetermined depressed temperature of the air, and means for causing air from said refrigerated zone to flow through at least a portion of said conduit when there is less than a predetermined buildup of frost on said cooling means, said last mentioned means responsive to restriction of said one end of the conduit by buildup of frost on said cooling means to direct a flow of chilled air onto said sensing means for lowering the air temperature sensed thereby to said predetermined depressed temperature, this last-said means including a passage for directing said flow of chilled air onto said sensing means.

2. A defrost system as set forth in claim 1 wherein said passage has one end in communication with said conduit and its other end adjacent said sensing means.

3. A defrost system as set forth in claim 2 further comprising an enclosure positioned within said refrigerated zone, said sensing means being located within said enclosure.

4. A defrost system as set forth in claim 3 wherein said one end of said conduit is spacedfrom said portion of the cooling means a predetermined distance thereby to form a gap, the build-up of frost on said portion of the cooling means reducing said gap and causing said restriction of said one end of the conduit.

5. A defrost system for refrigeration apparatus having cooling means for absorbing heat from a refrigerated zone, said cooling means being subject to frost buildup, said refrigeration apparatus having a flow path for the intake of air from said refrigerated zone and for passage of the air over said cooling means to chill the air and for the discharge of the chilled air into the refrigerated zone, and a blower for forcing the air through said path; said system comprising a heater for removing frost from said cooling means, and means for initiating operation of the heater in response to buildup of frost on the cooling means, said initiating means comprising a conduit providing communication between said flow path on the discharge side of the blower and said cooling means, one end of said conduit being a predetermined distance from said cooling means in close proximity thereto to form a gap, the buildup of frost on said portion of the cooling means reducing said gap and causing said restriction of said one end of the conduit, said blower forcing air through said conduit for outflow of air from said one end thereof and for impingement of said outflow on said portion of the cooling means, means for sensing air temperature adapted to energize said heater upon sensing a predetermined depressed temperature of the air, and means responsive to restriction of said one end of the conduit by build-up of frost on said cooling means to direct a flow of chilled air onto said sensing means for lowering the air temperature sensed thereby to said predetermined depressed temperature, this last-said means including a passage having one end in communication with said conduit and its other end adjacent said sensing means for directing said flow of chilled air onto said sensing means and further comprising an enclosure positioned within said refrigerated zone, said sensing means being located within said enclosure, said passage being in communication with said conduit at a point intermediate the ends of the conduit, the flow of air through the conduit for impingement on said portion of the cooling means causing air to flow from said refrigerated zone into said conduit via the enclosure and the passage, a buildup of frost on said portion of the cooling means at least partially restricting the outflow of air from said one end of the first conduit and thereby effecting a reversal of flow in said passage for delivery of chilled air into said enclosure for decreasing the air temperature therewith to said predetermined depressed temperature.

6. A defrost system as set forth in claim 4 wherein said cooling means comprises a coil of a refrigerant evaporator unit, said one end of said conduit being in close proximity to a portion of the coil and being spaced therefrom said predetermined distance.

7. A defrost system as set forth in claim 5 wherein said passage is constituted by a tube and the cross section of said conduit is substantially larger than the cross section of said tube.

8. A defrost system as set forth in claim 7 wherein said tube projects into said conduit and extends therewithin generally parallel to the conduit with the free end of the tube directed toward the one end of the conduit, the flow of air through said conduit toward its one end creating a suction within said tube thereby to draw air from said refrigerated zone via said enclosure and said tube.

9. A defrost system for refrigeration apparatus having cooling means for absorbing heat from a refrigerated zone, said cooling means being subject to frost buildup said refrigeration apparatus having a flow path for the intake of air from said refrigerated zone and for passage of the air over said cooling means to chill the air and for the discharge of the chilled air into the refrigerated zone, and a blower for forcing the air through said path; said system comprising a heater for removing frost from said cooling means, and means for initiating operation of the heater in response to buildup of frost on the cooling means, said initiating means comprising a conduit providing communication between said flow path on the discharge side of the blower and said cooling means, one end of said conduit being in close proximity to a portion of said cooling means, said blower forcing air through said conduit for outflow of air from said one end thereof and for impingement of said outflow on said portion of the cooling means, means for sensing air temperature adapted to energize said heater upon sensing a predetermined temperature of the air, and means responsive to restriction of said one end of the conduit by buildup of frost on said cooling means to direct a flow of chilled air onto said sensing means for lowering the air temperature sensed thereby to said predetermined depressed temperature, this last-said means including a passage having one end in communication with said conduit and its other end adjacent said sensing means for directing said flow of chilled air onto said sensing means and further comprising an enclosure positioned within said refrigerated zone including two refrigerated compartments, such as a food compartment and a freezer compartment in a combination refrigerator-freezer unit, one of said compartments being maintained at a warmer temperature than said other compartment, said enclosure being located within said warmer compartment, whereby with said cooling means substantially free of frost buildup and with air flowing through said conduit for impingement on said cooling means, relatively warm moist air from within said warmer compartment is drawn into said conduit via said enclosure and said passage, said warmer air mixing with said chilled air flowing in the conduit, said mixed air impinging on said portion of the cooling means thereby to cause frost buildup on said portion of the cooling means at a rate generally corresponding to the rate of frost buildup on the other portions of the cooling unit, said sensing means being located within said enclosure.

10. A defrost system as set forth in claim 1 wherein said refrigeration apparatus further includes a compressor, said temperature-sensing means providing for energization of said compressor and said blower when the air temperature sensed thereby is above said predetermined depressed temperature and providing for deenergization of said compressor and said blower and for energization of said heater upon sensing said predetermined depressed temperature.

11. A defrost system for a refrigerator having a food compartment, a freezer compartment, cooling means for absorbing heat from the compartments including a refrigeration system having an evaporator, a flow path for the intake of air from said food and freezer compartments and for passage of the air over said evaporator to chill the air and for the discharge of the chilled air into the food and freezer compartments, and a blower for forcing air through said path, said evaporator being subject to the build-up of frost thereon; said defrost system comprising a heater within said flow path for melting frost from the evaporator, a conduit having one end thereof constituting an inlet in communication with said flow path on the discharge side of said blower and having its other end constituting an outlet end in close proximity to a portion of said evaporator, said blower forcing chilled air through said conduit for the outflow of a stream of air from said outlet end and for impingement of said air stream on a portion of said evaporator, an enclosure, an opening in said enclosure providing communication between said food compartment and said enclosure, a tube extending between said conduit intermediate its ends and said en- 10 mixed air flow impinging on said portion of the evaporator thereby to cause frost build-up on said portion of the evaporator, said build-up of frost restricting the outflow of air from said conduit, this restriction causing a decrease in said suction at the free end of the tube and an increase in back pressure within said conduit which cause a reversal of flow in said tube whereby chilled air is delivered to said enclosure via said tube thereby to lower the air temperature therewithin to said predetermined depressed temperature. 

1. A defrost system for refrigeration apparatus having cooling means for absorbing heat from a refrigerated zone, said cooling means being subject to frost buildup, said refrigeration apparatus having a flow path for the intake of air from said refrigerated zone and for passage of the air over said cooling means to chill the air and for the discharge of the chilled air into the refrigerated zone, and a blower for forcing the air through said path; said system comprising a heater for removing frost from said cooling means, and means for initiating operation of the heater in response to buildup of frost on the cooling means, said initiating means comprising a conduit providing communication between said flow path on the discharge side of the blower and said cooling means, one end of said conduit being in close proximity to a portion of said cooling means, said blower forcing air through said conduit for outflow of air from said one end thereof and for impingement of said outflow on said pOrtion of the cooling means, means for sensing air temperature adapted to energize said heater upon sensing a predetermined depressed temperature of the air, and means for causing air from said refrigerated zone to flow through at least a portion of said conduit when there is less than a predetermined buildup of frost on said cooling means, said last mentioned means responsive to restriction of said one end of the conduit by buildup of frost on said cooling means to direct a flow of chilled air onto said sensing means for lowering the air temperature sensed thereby to said predetermined depressed temperature, this last-said means including a passage for directing said flow of chilled air onto said sensing means.
 2. A defrost system as set forth in claim 1 wherein said passage has one end in communication with said conduit and its other end adjacent said sensing means.
 3. A defrost system as set forth in claim 2 further comprising an enclosure positioned within said refrigerated zone, said sensing means being located within said enclosure.
 4. A defrost system as set forth in claim 3 wherein said one end of said conduit is spaced from said portion of the cooling means a predetermined distance thereby to form a gap, the build-up of frost on said portion of the cooling means reducing said gap and causing said restriction of said one end of the conduit.
 5. A defrost system for refrigeration apparatus having cooling means for absorbing heat from a refrigerated zone, said cooling means being subject to frost buildup, said refrigeration apparatus having a flow path for the intake of air from said refrigerated zone and for passage of the air over said cooling means to chill the air and for the discharge of the chilled air into the refrigerated zone, and a blower for forcing the air through said path; said system comprising a heater for removing frost from said cooling means, and means for initiating operation of the heater in response to buildup of frost on the cooling means, said initiating means comprising a conduit providing communication between said flow path on the discharge side of the blower and said cooling means, one end of said conduit being a predetermined distance from said cooling means in close proximity thereto to form a gap, the buildup of frost on said portion of the cooling means reducing said gap and causing said restriction of said one end of the conduit, said blower forcing air through said conduit for outflow of air from said one end thereof and for impingement of said outflow on said portion of the cooling means, means for sensing air temperature adapted to energize said heater upon sensing a predetermined depressed temperature of the air, and means responsive to restriction of said one end of the conduit by build-up of frost on said cooling means to direct a flow of chilled air onto said sensing means for lowering the air temperature sensed thereby to said predetermined depressed temperature, this last-said means including a passage having one end in communication with said conduit and its other end adjacent said sensing means for directing said flow of chilled air onto said sensing means and further comprising an enclosure positioned within said refrigerated zone, said sensing means being located within said enclosure, said passage being in communication with said conduit at a point intermediate the ends of the conduit, the flow of air through the conduit for impingement on said portion of the cooling means causing air to flow from said refrigerated zone into said conduit via the enclosure and the passage, a buildup of frost on said portion of the cooling means at least partially restricting the outflow of air from said one end of the first conduit and thereby effecting a reversal of flow in said passage for delivery of chilled air into said enclosure for decreasing the air temperature therewith to said predetermined depressed temperature.
 6. A defrost system as set forth in claim 4 wherein said cooling means comprises a coil of A refrigerant evaporator unit, said one end of said conduit being in close proximity to a portion of the coil and being spaced therefrom said predetermined distance.
 7. A defrost system as set forth in claim 5 wherein said passage is constituted by a tube and the cross section of said conduit is substantially larger than the cross section of said tube.
 8. A defrost system as set forth in claim 7 wherein said tube projects into said conduit and extends therewithin generally parallel to the conduit with the free end of the tube directed toward the one end of the conduit, the flow of air through said conduit toward its one end creating a suction within said tube thereby to draw air from said refrigerated zone via said enclosure and said tube.
 9. A defrost system for refrigeration apparatus having cooling means for absorbing heat from a refrigerated zone, said cooling means being subject to frost buildup said refrigeration apparatus having a flow path for the intake of air from said refrigerated zone and for passage of the air over said cooling means to chill the air and for the discharge of the chilled air into the refrigerated zone, and a blower for forcing the air through said path; said system comprising a heater for removing frost from said cooling means, and means for initiating operation of the heater in response to buildup of frost on the cooling means, said initiating means comprising a conduit providing communication between said flow path on the discharge side of the blower and said cooling means, one end of said conduit being in close proximity to a portion of said cooling means, said blower forcing air through said conduit for outflow of air from said one end thereof and for impingement of said outflow on said portion of the cooling means, means for sensing air temperature adapted to energize said heater upon sensing a predetermined temperature of the air, and means responsive to restriction of said one end of the conduit by buildup of frost on said cooling means to direct a flow of chilled air onto said sensing means for lowering the air temperature sensed thereby to said predetermined depressed temperature, this last-said means including a passage having one end in communication with said conduit and its other end adjacent said sensing means for directing said flow of chilled air onto said sensing means and further comprising an enclosure positioned within said refrigerated zone including two refrigerated compartments, such as a food compartment and a freezer compartment in a combination refrigerator-freezer unit, one of said compartments being maintained at a warmer temperature than said other compartment, said enclosure being located within said warmer compartment, whereby with said cooling means substantially free of frost buildup and with air flowing through said conduit for impingement on said cooling means, relatively warm moist air from within said warmer compartment is drawn into said conduit via said enclosure and said passage, said warmer air mixing with said chilled air flowing in the conduit, said mixed air impinging on said portion of the cooling means thereby to cause frost buildup on said portion of the cooling means at a rate generally corresponding to the rate of frost buildup on the other portions of the cooling unit, said sensing means being located within said enclosure.
 10. A defrost system as set forth in claim 1 wherein said refrigeration apparatus further includes a compressor, said temperature-sensing means providing for energization of said compressor and said blower when the air temperature sensed thereby is above said predetermined depressed temperature and providing for deenergization of said compressor and said blower and for energization of said heater upon sensing said predetermined depressed temperature.
 11. A defrost system for a refrigerator having a food compartment, a freezer compartment, cooling means for absorbing heat from the compartments including a refrigeration system having an evaporator, a Flow path for the intake of air from said food and freezer compartments and for passage of the air over said evaporator to chill the air and for the discharge of the chilled air into the food and freezer compartments, and a blower for forcing air through said path, said evaporator being subject to the build-up of frost thereon; said defrost system comprising a heater within said flow path for melting frost from the evaporator, a conduit having one end thereof constituting an inlet in communication with said flow path on the discharge side of said blower and having its other end constituting an outlet end in close proximity to a portion of said evaporator, said blower forcing chilled air through said conduit for the outflow of a stream of air from said outlet end and for impingement of said air stream on a portion of said evaporator, an enclosure, an opening in said enclosure providing communication between said food compartment and said enclosure, a tube extending between said conduit intermediate its ends and said enclosure, and means for sensing the air temperature within said enclosure adapted to energize the heater upon sensing a predetermined depressed temperature, said tube projecting into said conduit and extending therewithin generally parallel to the conduit with its free end facing toward the outlet end of the conduit, said flow of chilled air through said conduit creating a suction at the free end of said tube thereby to draw relatively warm air from the food compartment into said conduit via said enclosure and said tube, said warm air mixing with said chilled air in said conduit and said mixed air flow impinging on said portion of the evaporator thereby to cause frost build-up on said portion of the evaporator, said build-up of frost restricting the outflow of air from said conduit, this restriction causing a decrease in said suction at the free end of the tube and an increase in back pressure within said conduit which cause a reversal of flow in said tube whereby chilled air is delivered to said enclosure via said tube thereby to lower the air temperature therewithin to said predetermined depressed temperature. 